As a lifelong Science Fiction fan, I have seen nearly every episode of nearly every show. By far, the single episode of any series with the greatest emotional impact is "Sleeping in Light". I have seen it countless times, and every single time it brings me to tears. Even in a series known as a masterpiece of storytelling, it goes beyond the other episodes to be an incredibly heart-wrenching finale. I believe it is unmatched in television science fiction, and possibly in TV in general.
As has been said before, it is imperative that you watch the rest of the series, in its entirety and in order, before you watch the finale. Otherwise it will lose most of its effect, and possibly spoil some of the rest of the show.
As has been said before, it is imperative that you watch the rest of the series, in its entirety and in order, before you watch the finale. Otherwise it will lose most of its effect, and possibly spoil some of the rest of the show.